Tertiary mid-chain radicals

Backbiting generates a tertiary (mid-chain) radical, R, by abstraction of a hydrogen atom from an acrylate unit on the backbone of the secondary (chain-end) radical R, most likely via the formation of a six-membered ring, as shown in Scheme 4.7 for BA. Subsequent addition of monomer to R creates a short-chain branch (SCB) in the polymer and leads to re-formation of a chain-end radical. The propagation rate coefficient for monomer addition to the MCR, kpt, is significantly lower than that for addition to the secondary chain end [10]. 

To begin backbiting is accounted for via inclusion of the reaction steps for tertiary radical, formation (la), propagation (Ib) and termination (Ic and Id) into the PREDICI simulation (see Scheme 1). These reactions depend on the rate coefficients for backbiting, febb, tertiary radical propagation, i and tertiary radical termination, which occurs either between two tertiary radicals, o " between a mid-chain and an end-chain radical, where the moiety X represents the continuing chain. Additionally, the model was expanded to account for the reactions for the RAFT... 

Many studies have been devoted to the determination of the structures of radicals and to the changes in their concentrations during cross-linking photopolymerizations. Most of these studies focused on the structures of the trapped radicals. In the case of multiacrylates, generally a 3-line spectrum was observed, which was consistent both with the chain-end structure (II) as well as with a mid-ehain structure (III) obtained after tertiary hydrogen abstraction from the polymer chain ... 

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